1. Field of the Invention
The present invention relates to hypodermic syringes having structure to help prevent accidental needle sticks. More particularly, the present invention relates to a safety needle syringe capable of withdrawing the hypodermic needle into the syringe barrel after use.
2. Description of Related Information
Generally speaking, a syringe includes a cylindrical barrel, commonly made of thermoplastic material or glass, having a distal end connected to a sharpened needle cannula and a proximal end adapted to receive a stopper and plunger assembly.
In recent years there has developed an increased concern regarding the transfer of disease, infection or the like to syringe users and health care professionals who accidentally, or through negligent handling, stick themselves with hypodermic needles while disposing of used hypodermic syringe products. In many areas in a hospital, where needle cannula products are used, disposal bins are provided so that a syringe or other needle cannula product may be immediately discarded in a safe rigid container. However, there are areas of medical practice such as emergency rooms, or other areas where disposal containers are not readily available or practical, where products having self-contained safety features are desirable. In theory, after such a syringe is used to inject medication or for another purpose, a safety device contained within the syringe is activated to prevent further contact with the sharp needle tip. One type of safety syringe includes structure which allows the withdrawal of the hypodermic needle into the syringe barrel to minimize the chance of further contact with the sharp needle tip. The syringe, in this condition, can be more safely transported to a disposal system.
Such a syringe is taught in U.S. Pat. No. 4,026,287. This patent teaches a syringe having a frangible zone which allows separation of the forward wall of the barrel, which is connected to the hypodermic needle, from the side wall of the barrel. The syringe also contains structure on the interior of the forward wall and the exterior of the piston for selectively attaching the piston to the forward wall so that the user can forcibly twist the piston to break the frangible means and draw the forward wall, including the hypodermic needle, into the syringe barrel. This design requires a compromise in the design of the syringe barrel. The barrel must be strong enough to remain intact during normal use yet weak enough to be sheared apart by any user regard less of strength.
U.S. Pat. No. 3,828,775 teaches a retracting needle syringe wherein the needle assembly is contained within the syringe barrel before use and is withdrawn back into the barrel after use. Designs of this type have a disadvantage in that the user cannot change needle size and the decision regarding what size needle will be used with the syringe must be made at the time the syringe is purchased.
In many situations, the decision regarding which needle to use is made at the time of injection. More viscous medications may require a large needle. Less viscous medications may be delivered with a smaller needle which is believed to be less painful. Also, the depth of injection may be different depending on the therapy and the portion of the body being injected.
U.S. Pat. No. 4,675,005 teaches a retracting needle syringe wherein the needle is held at the distal end of the barrel through interaction of mating threads on the exterior of the needle hub and the interior of the distal barrel opening. This syringe relies on a torque or rotational force supplied by the user to secure the needle in an extended position and reversal of this procedure to withdraw the needle back into the barrel. Like many designs, including those wherein the needle is retained by a frictional interference fit, this design requires substantially the same force to install as to remove. Accordingly, the needle must be secure enough to withstand normal use which may include puncturing rubber vial stoppers, and still be easy enough to unsecure by any user. Without a lock or additional structure the holding force is, in these many designs, substantially equal to the removal or withdrawal force. Also, in designs which rely on rotational force applied through the plunger rod to tightly secure the needle in the extended position, the user must be careful not to aggressively turn the plunger rod in the wrong direction when attempting to loosen the needle. In this case the needle will be more tightly secured and thus more difficult to remove.
Although the prior art teaches many useful and different syringe assemblies having the capacity to withdraw the needle into the syringe barrel after use, there still exists a need for a simple, straight-forward, reliable, easily fabricated safety needle syringe which allows the user to change needles at the time of use and provides structure to withdraw the needle into the syringe barrel using substantially axial forces such as those used for drawing liquid into a syringe. There also exists the need for a safety needle syringe having locking structure for holding a needle in a position with respect to the barrel, which can be deactivated to lower the force required for withdrawal of the needle into the barrel.